I assume this is like the old Brit Roto-kegs that some of the old timers here may have dabbled with. Those dealies didn't use CO2 though, they just relied on natural conditioning and gravity to move beer. The one I had a long time ago was not as big as 5 gallons I don't think, because I always bottled some and kegged some for each batch. Like simple polypin containers some people have also played with, the roto-keg was only good for low-carbonated Brit-style beers. Maybe they're more of historical interest, but both of the old Dave Line books have pictures showing this kind of gear.

Yep, they are the large thick plastic kegs as described in the Dave Line books,still very popular in England, the newer ones come with an adaptor on the lid to fit small 8oz or 16oz CO2 cylinders to push beer out once natural carbonation has lost its pressure.Some draw the beer from the top via afloat system ensuring clear pints each time whilst others pour from the bottom,Id guess that this is a top style one hence the co2 injection.They are rather bulky to fit in the fridge.My father used one for years until he saw my cornelius set up and that was enough for him to ditch his old keg and go over to Cornelius, much easier and convenient.
Fraoch